Smart metering brings intelligence, connectivity and data protection to energy and resource management
Smart metering brings intelligence, connectivity and
data protection to energy and resource management
By Keith Odland, MCU Marketing Manager, Silicon Labs
As green energy management becomes a global imperative, the idea of implementing intelligent systems and wireless technology to more efficiently use energy and other natural resources has become a pervasive reality. Smart metering began with a relatively simple concept. If you add embedded intelligence and a communications link to a traditional metering device, you gain the ability to remotely access the data that the smart meter has collected. The first and most obvious application of this concept was to streamline the meter reading and billing process. However, through this seemingly simple enhancement of a communications link, a network was born, and with it came an explosion of applications and innovations transforming the way energy is measured, priced and consumed.
Multiple forms of energy are monitored and controlled in the energy metering ecosystem, including gas, water, electricity and thermal energy. Metering information from a group of residential, commercial and industrial facilities is typically sampled at regular intervals and aggregated by a common metering collector before being sent to the service provider. While the ability to automatically read meters provides compelling advantages, utilities today are implementing two-way networks that further allow individuals and businesses to make more efficient use of the energy they consume. In-home energy displays, thermostats and load controllers are emerging tools in the marketplace that enable this improved level of control. Operators equally enjoy the benefits of two-way networking due to the improved reliability and dynamic billing opportunities that it provides.
Primary utility meter types
In the very broadest sense, there are three distinct categories of metering devices. The most common is the electricity meter, which quantifies the consumption of electrical energy. The second most common is a meter that measures the consumption of a fluid, such as water, natural gas, or fuel oil. The third category of meters quantifies the consumption of thermal energy, commonly referred to as heat meters or heat cost allocaters.
With respect to electricity meters, functionality can be split into two functional areas: the metrology or measurement function and the communications subsystem. The requirements for the metrology function vary by region and meter type (residential versus industrial), with some of the variables including the number of phases being measured, the accuracy of the measurement, the requirement for different rates dependent on time of use and the level of security (including encryption) required at the communication layer. In general, these systems measure the electrical power consumed by a customer, the power factor of the load and the time of electricity consumption to support multi-rate metering. These measurements rely on various sensor technologies that match the number of electrical phases in the system. Consumer meters are typically single phase, whereas commercial and industrial customers might have multiphase meters. These meters usually derive power from the mains but require an alternate supply, such as a battery or supercapacitor to maintain a state in disconnect or disruption of service condition.
Gas and water meters (see Figure 1) are generally battery-powered and consist of an embedded controller that interfaces to a metering sensor, display and communications block, typically a wireless transmitter or transceiver. They often use positive displacement flow meters to measure the number of times a unit volume of the fluid moves through the meter. For more viscous fluids, volume is measured by a magnet or shaft that rotates. Each revolution is converted to an electrical signal and accumulated by an embedded controller. Less viscous fluids such as natural gas might rely on ultrasonic transducers to measure mass flow. Regardless of the material that is measured, low power consumption is a critical design parameter in these systems, as electricity is generally not wired to the locations of these meters.
Figure 1: Example of a smart gas/water meter system based on the Si10xx wireless MCU.
The third class of meters is the thermal energy meter. Heat meters and heat cost allocaters are typically installed in buildings with multiple residences that rely on a centralized heating system. These meters measure the amount of heat being delivered to a location in a given period of time. Again, these are battery-operated solutions that are optimized for the lowest overall system power.
Similar in topology to gas and water meters, thermal energy meters generally have an embedded controller measuring the flow and temperature of the heating fluid and incorporate a display and communications block. Heat is billed by the power (thermal energy per unit time) delivered to the location measured by the flow of the heating fluid and the input and exit fluid temperatures over a given time period. This information is displayed for the customer on a display either integrated into the meter itself or remotely located. This information from multiple locations is transmitted over a wireless link to a collector, where it is aggregated and communicated to the service provider.
Metering functions
- Quantitative measurement: This varies by meter type, but the primary function of any meter is to accurately measure a quantity of something. These measurement systems span a wide range of topologies. Some examples include, but are not limited to: temperature sensors, flow sensors, shunt resistors, isolation transformers, current transformers and time-keeping systems.
- Control and calibration: This also varies by type of meter and is typically needed to compensate for small variations in the measurement system. These systems can also perform functions such as tamper resistance and interruption of service.
- Communications: This can be used to configure parameters in the meter and transfer stored data to a host via wired or wireless connection. It can also be used to update the firmware or other operational characteristics of the meter.
- Power management: Low power and system robustness are needed in the event of a primary source of energy going down. In nonelectric metering applications, power management is critical to minimize power consumption and maximize the battery service interval.
- Display: Interfaces to low-cost and low-power LCD and LED displays in seven-segment, alphanumeric, or matrix format are very common user interfaces. In many cases, there is a regulatory requirement that a customer must have the ability to view the billable quantity directly from the meter.
- Synchronization: Timing synchronization is critical for the reliable transmission of data to central hub or other collector systems to support functions such as data analysis and accurate billing. This is particularly needed in a wireless network that has an unpredictable or asynchronous communication protocol.
Savings from ultra-low-power technology
In some applications and markets, meters are subject to stringent low-power requirements. For example, the service interval for an underground water meter is 20 years or more. For these applications, specific lithium battery chemistries (such as lithium thionyl chloride or Li-SOCl2) with very low self-discharge rates are needed to meet the longevity requirement. Unfortunately, these battery chemistries can be quite costly compared to their more established counterparts.
A typical “D” size Li-SOCl2 battery has a capacity between 16 and 19 amp-hours. Even in high volume, costs of 50 cents to 75 cents per amp-hour are not unreasonable. However, when we consider the entire service life of 20 years, steady-state system currents of 10 microamps can result in dollars of battery cost for the meter provider:
10 microamps * 24 hour/day * 365 day/year * 20 year * $0.75 amp/hour = $1.31 in battery (and system) cost
Meter manufacturers frequently produce and ship millions of units per year. The high level of smart meter shipments clearly illustrates the critical importance of optimizing metering systems for the lowest power consumption.
Bringing embedded intelligence to bear
When we review the different meter categories and overlay them on the metering functions, a set of core capabilities and technologies surface as key building blocks for designers of these systems. The key to any embedded intelligent system is a microcontroller (MCU). In these applications, the MCU should have very low-power requirements as well as integrating features such as a real-time clock, analog-to-digital converter and communications interface. More advanced features such as integrated LCD controllers, a cyclic redundancy check block, or an encryption engine can reduce the processing burden of the MCU and enable it to reside in low-power modes for longer periods of time, ultimately reducing the overall system power consumption.
Figure 2: Silicon Labs Si10xx wireless MCU provides a highly integrated control and wireless connectivity solution.
Wireless transmitters, receivers and transceivers are becoming more common in these systems. Key features include high levels of integration, very low power operation, fast start-up from low-power states, high receiver sensitivity (greater than -118 dBm) and high transmit powers without external power amplifiers (up to 20 dBm). More advanced features include automatic packet handling, integrated FIFO and variable frequency and modulation schemes.
Wireless MCUs (see example in Figure 2) that combine the MCU function with a wireless transceiver are also available for use in smart meter applications. These highly integrated single-chip devices can help reduce BOM and system cost while providing a low-power embedded control solution capable of high-performance wireless connectivity.
Other technologies key to these next-generation metering systems include wired access products such as modems for line-based data communication, timing solutions for network synchronization and isolation products for safety, compliance and data protection for smart electrical meters.
Protecting smart meter data
Although smart meters are more sophisticated than electromechanical power meters, a primary concern in smart meter design is measurement data integrity, which can directly impact the utility provider’s billing revenue. One of the most effective solutions for ensuring data integrity in smart meter designs is the use of state-of-the-art digital isolation technology.
Smart power meters use galvanic isolation to protect internal low-voltage integrated circuits, as well as utility service personnel, from exposure to the high-voltage mains. In wired metering applications, such as those deployed in high-density residential complexes, isolation also may be used between the controller and the digital data bus, as shown in Figure 3.
Figure 3: Smart power meter with digital communications bus
Other subsystems, especially those that are exposed to high voltages, also must contain isolation circuitry. For instance, galvanic isolation is necessary between an internal smart meter controller IC and a power line communications (PLC) modem. Signal isolation in these systems may be implemented in a number of ways.
Optocouplers are often used in smart meters for signal isolation, but their usage presents design challenges. The primary drawback is the limited common mode transient immunity (CMTI) of optocouplers. CMTI is a measure of the isolator’s ability to reject fast transient noise signals that are present between the input and the output sides of the isolation barrier. Because of their physical structure, optocouplers tend to have high parasitic input-output capacitance (typically in picofarads). Higher internal parasitic coupling capacitance results in poorer CMTI performance. Optocoupler suppliers often recommend overdriving the optocoupler’s LED to increase noise immunity when on, and reverse-biasing the LED for added immunity when off. These actions increase optocoupler CMTI, but they decrease the device lifetime, which in turn negatively affects system reliability and drives up maintenance costs.
Another isolation solution for smart power meter applications involves the use of an isolation transformer. However, transformers are generally avoided because of their susceptibility to data-corrupting electromagnetic interference (EMI). Pulse transformers multiply this concern because of their inherently wider bandwidth, which is necessary to faithfully convey digital signals.
Electromagnetic (EM) immunity is a primary concern in power meter designs for two reasons. First, there is a high probability that the meter will be installed in an electromagnetically noisy location. Second, some isolation techniques are potentially the weakest point in the meter system for exploitation. For example, the application of an external field to a transformer-based system can negatively impact data integrity. Indeed, there have been cases of utility customers disabling power meters by attaching strong magnets or coils to the equipment. In either case, the external magnetic field or EM noise will present false measurement data to the controller.
Modern CMOS digital isolators address these concerns in smart meter applications. Compared to optocouplers, CMOS-based digital isolators deliver substantially higher CMTI performance while maintaining higher operating lifetimes and greater reliability. For example, Silicon Labs’ Si84xx family of CMOS digital isolators has a typical CMTI specification of 25 kV/µs, and next-generation isolation devices are expected to double this performance level.
CMOS digital isolators are vastly superior to alternative isolation technologies in terms of electromagnetic performance. The Si84xx isolators, for example, exhibit the highest EMI tolerance (>300 V/m E-field immunity, and >1000 A/m magnetic field immunity) of all commercially-available digital isolation devices. These digital isolators achieve this performance by using a differential signal path to transmit data across the isolation barrier. Paired with narrow pass-band filtering, this provides superior common mode noise rejection, as shown in Figure 4.
Figure 4: Differential signals and a narrowband receiver reject common mode noise in digital isolators.
In addition, a CMOS-based isolator implementation minimizes device feature sizes, which helps prevent the isolator from acting as an antenna for stray fields. Avoiding the use of transformers enables the system to maintain a high level of magnetic immunity.
As smart meters become prevalent in the market with the worldwide build-out of the smart grid, meter installers will become less discriminating about the environments in which the meters are located, increasing the probability of measurement data corruption. Any metering component used in the meter design that can be adversely affected by electrical noise or electromagnetic fields must be considered to be a weak link in the overall integrity of the system. These components have the potential to disrupt data to the smart meter controller and ultimately invalidate the utility’s billing information.
Despite the popularity of optocouplers and transformers as isolation technologies, both of these solutions have weaknesses that should cause concern for metering applications. CMOS digital isolators offer the optimal isolation solution for smart meters by providing superior immunity to electrical noise and external fields. Using CMOS digital isolators in smart meters ensures that accurate, uncorrupted power measurement data passes across the isolation barrier to the system controller.
In summary, as more embedded intelligence, connectivity options and data protection capabilities are integrated into utility meters, there is no doubt there will be a growth of green-energy applications and additional opportunities that will harness the myriad benefits of smart meters.
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